1. Field of the Invention
The present invention relates to a power meter, and particularly to a three-phase clamp-type power meter in the form of a hand tool suitable for measuring 3-phase, 3-line or 3-phase, 4-line AC/DC unbalanced power, and has the advantages of low cost, light weight, memory, dual-numeral display, etc.
2. Description of the Prior Art
Measuring voltage, current or power frequently occurs in daily life as well as in industry so that various kinds of electric meters have been developed as needed, for example, the widely-used multimeter which was developed mainly for simple measurements. However, the multimeter is only suitable for measuring small ranges of voltage and current. For measuring a high voltage or current, using the multimeter becomes quite dangerous and, in fact, some circumstances do not allow the user to perform measurements simply by test lines.
Furthermore, for subscribers requiring high power, it is important to frequently monitor the power factor, that is, the ratio between the real power actually used and the input apparent power, to enhance the performance of their internal circuitry and to save on power consumption. Thus, there is a need of an excellent and reliable power meter for users to measure the power factor, particularly in the high power environments.
Digital electric meters have been developed and are capable of displaying the measured data on a display such as an LCD (Liquid Crystal Display). However, just like the multimeter, most of the digital electric meters have been developed for low voltage applications or simple requirements and, thus, the range of applicable use is limited.
An example is given below to further explain existing electric meters. As shown in FIG. 9, the PA-2000 power analyzer 50 produced by Musashi Inc., Japan, is described below. The power analyzer 50 essentially includes: an operating unit (not shown) inside the analyzer 50; voltage test line assemblies 51, 52; current test line assemblies 53, 54; temperature-sensing terminals 55; a display 56; an RS-232 interface port 57; a power switch 58; a printing device 59; and a handle 501 provided on the front end of the power analyzer 50 for the user to carry the analyzer 50.
One voltage test line assembly 51 comprises three lines for measuring a 3-phase, 3-line voltage, and the other voltage test line assembly 52 comprises four lines for measuring a 3-phase, 4-line voltage. One current test line assembly 53 comprises two lines for measuring a 3-phase, 3-line current, and the other current test line assembly 54 comprises three lines for measuring a 3-phase, 4-line current.
In the case of measuring a 3-phase, 4-line power, it is necessary to use the three lines of the current test line assembly 54 to respectively clamp the three power lines respectively of R, S, T phases to measure the currents flowing therein, and to connect the four lines of the voltage test line assembly 52 to the power lines respectively of R, S, T phases as well as the ground line to measure the voltages on the R, S and T-phased lines. Then, the internal operating unit of the analyzer 50 calculates each phase's real power (KW.sub.R, KW.sub.S and KW.sub.T) by the equation P(power)=I(current) * V(voltage)*COS .theta. (.theta. is the phase angle between the voltage and current), so that the 3-phase, 4-line real power KW.sub.3.phi.4W can be obtained as below: EQU KW.sub.3.phi.4W =KW.sub.R +KW.sub.S +KW.sub.T
The result of the measurement is displayed on the display 56, and the printing device 59 can print out the measurement results.
However, the power analyzer 50 is too bulky to be an effective hand tool. The components and wiring of the analyzer 50 are also too complex, and sometimes the test line assemblies need more than one person to set them up and hold them in place during the test.
In addition, the power analyzer 50 lacks a memory capability. This is particularly inconvenient when a test requires several measurements. Also, the power analyzer 50 cannot simultaneously display dual numerals such as a voltage value plus a current value. The measurable range of the voltage, current and power of the power analyzer 50 are also insufficient.
Accordingly, there is a need for a power meter capable of measuring 3-phase, 3-line or 3-phase, 4-line power lines and the AC/DC unbalanced power on power lines with low cost, light weight, a memory capability, a dual-value display, large measurable ranges, etc.